The invention relates to a holding device as used in vacuum units. It is used, for example in an intermediate chamber after transfer of a disc-shaped workpiece from a transport mechanism in a transfer position in which a lifting table thereof is, for example, in a lower end position, for lifting said workpiece into a processing position by virtue of the fact that the lifting table performs a lifting movement to an upper end position. In the processing position, the top of the workpiece is present in a process chamber and can be subjected to a processing step. After processing is complete, the lifting table lowers the workpiece again to the transfer position, where it can once again be picked up by the transport mechanism. A corresponding vacuum unit is described in EP-A-0 343 530.
In the case of holding devices of this type, it is desired that the support ring on which the workpiece is placed in the transport position is centred as accurately as possible relative to the support plate on which said workpiece rests in the processing position and by means of which it is heated or cooled. Consequently, the recesses at the edge of the support plate, which are necessary for supporting the support ring, can be kept small, and the processing of the workpiece, such as coating, heating or cooling or any other action of the support plate on the workpiece, takes place in a uniform manner.
A known holding device of the generic type is described in EP-A-0 452 779. There, the support ring is in the form of a ring directly surrounding the support plate. Since a specific radial distance must be provided between the two because of any different thermal expansions, exact centring of the support ring with respect to the axis of the support plate is not provided for in this design. Since the clamping ring is centred with respect to the support ring, it too is inevitably inaccurately centred with respect to the support plate.
The invention is intended to provide a remedy here. The invention, as characterized in the Claims, provides a holding device in which support plate, support ring and clamping ring are centred with respect to a common axis through a common centring device which engages all of said parts and whose function is not impaired by different thermal expansions of said parts, and which does not hinder necessary mutual axial displacements thereof.
A fundamental principle of operation of the proposed corresponding centring insensitive to thermal expansion is described for holding a lens in RU-C-2 047 197, where radial lamellae anchored to a housing engage corresponding radial grooves in the lens mount. In the case of this holder, however, no mutual axial displacement of the parts is envisaged.
The advantages achieved by the invention are in particular that concentric positioning of the disc-shaped workpiece on the support plate can be ensured and, owing to the accurate centring of the clamping ring with respect to the support plate and hence of the workpiece, the edge waste is very small. The holding device is particularly suitable for workpieces such as storage discs, for example hard discs, magnetooptical storage discs, CDs and especially semiconductor wafers.